1. Field of the Invention
The present invention relates generally to a communication system employing HSDPA (High Speed Downlink Packet Access), and in particular, to a method for resetting a MAC-hs (Medium Access Control-high speed) layer entity upon reset of an RLC (Radio Link Control) layer entity.
2. Description of the Related Art
HSDPA refers to an apparatus, method and system for managing HS-DSCHs (High Speed Downlink Shared Channels), i.e., downlink data channels for supporting high-speed downlink packet data transmission, and control channels related thereto in a W-CDMA (Wideband Code Division Multiple Access) communication system. In order to support high-speed packet data transmission, a communication system using the HSDPA has newly introduced an AMC (Adaptive Modulation and Coding) scheme, an HARQ (Hybrid Automatic Retransmission Request) scheme, and an FCS (Fast Cell Select) scheme.
1) AMC Scheme
The AMC scheme is a data transmission scheme for adaptively determining a modulation mode and a coding mode of different data channels according to a channel condition between a Node B and a UE (User Equipment), thereby to increase the overall utilization efficiency of a cell. The AMC scheme has have a plurality of modulation modes and a plurality of coding modes, and modulates and codes data channels by combining the modulation modes and the coding modes. Generally, each of combinations of the modulation modes and the coding modes is called “MCS (Modulation and Coding Scheme)”, and there are a plurality of MCSs with level #1 to level #N according to the number of combinations of the modulation modes and the coding modes. In other words, the AMC scheme adaptively determines a level of the MCS according to a channel condition between the Node B and the UE currently wirelessly connected to the Node B, thereby increasing the overall efficiency of the Node B.
2) HARQ Scheme
The HARQ, or N-channel SAW HARQ (N-channel Stop And Wait Hybrid Automatic Retransmission Request) scheme, will be described. In the typical ARQ (Automatic Retransmission Request) scheme, an ACK(acknowledgement signal) and retransmission packet data are exchanged between an UE and an RNC (Radio Network Controller). However, in the HARQ scheme, there have been proposed the following two plans to increase efficiency of the ARQ scheme. In a first plan, the HARQ exchanges data retransmission request and response between the UE and the Node B. In a second plan, the HARQ temporarily stores errored data and then combines it with retransmission data of the corresponding data before transmission. Further, the HARQ scheme exchanges ACK and retransmission packet data between the UE and the MAC of the Node B over the HS-DSCH. In addition, the HSDPA scheme has introduced the N-channel SAW HARQ scheme in order to make up for the shortcomings of the conventional SAW ARQ (Stop And Wait ARQ)scheme. The SAW ARQ scheme transmits the next packet data only after receiving ACK for the previous packet data. Therefore, in some cases, the SAW ARQ scheme should await ACK, though it can presently transmit packet data. The N-channel SAW HARQ scheme increases channel utilization efficiency by continuously transmitting a plurality of data packets before receiving the ACK for the previous packet data. That is, if it is possible to establish N logical channels between the UE and the Node B and identify the N channels according to timing and channel numbers, the UE receiving the packet data can recognize the channel through which the received packet data was transmitted at a certain time, and take necessary measures of, for example, reconfiguring the received packet data in a desired order or soft-combining the corresponding packet data.
3) FCS Scheme
The FCS scheme rapidly selects a cell having a good channel condition among a plurality of cells, when a UE receiving an HSDPA service enters a cell-overlapped region, i.e., a soft handover region. To be specific, if the UE receiving the HSDPA service enters a cell-overlapped region between a first Node B and a second Node B, then the UE establishes radio links to a plurality of the cells, i.e., a plurality of Node Bs. Here, a set of the cells, to which the radio links are established by the UE, is called an “active set”. The FCS scheme receives HSDPA packet data from only the cell maintaining the best channel condition among the cells included in the active set, thereby to reduce the overall interference. Here, a cell transmitting the HSDPA packet data for its best channel condition among the cells in the active set is called a “best cell”. The UE periodically checks the channel conditions with the cells belonging to the active set. Upon detecting a cell having a channel condition better than that of the current best cell, the UE transmits a best cell indicator to all of the cells in the active set in order to replace the current best cell with a new best cell. The best cell indicator includes an identifier of the selected new best cell. Upon receiving the best cell indicator, the cells belonging to the active set analyze the cell identifier included in the received best cell indicator to determine whether the received best cell indicator is destined for them. The selected best cell transmits packet data to the UE using a HS-DSCH.
Next, a process of resetting an RLC layer in a general communication system employing the HSDPA will be described with reference to FIGS. 1 and 2. In the following description, a MAC layer entity and an RLC layer entity will be referred to as “MAC” and “RLC” for short.
FIG. 1 illustrates a process of resetting an RLC in a general CDMA communication system not employing the HSDPA. Specifically, FIG. 1 illustrates a process of resetting the RLC operating in an AM(acknowledged mode).
Generally, in the system not employing the HSDPA, the RLC manages retransmission of errored data, and the MAC and the physical layer do not participate in the retransmission. However, since the HSDPA applies the HARQ function to the physical layer, the physical layer performs a retransmission control function caused by occurrence of an error, independent of the RLC. An operation of the RLC will be described herein below. An operation mode of the RLC is divided into a TM(transparent mode), an UM(unacknowledged mode), and an AM. The HSDPA operates only in the UM and the AM.
First, a description will be made of an operation of the RLC in the UM. When the UE and the Node B perform retransmission to the RLC in the UM, a sender RLC inserts a header with a SN(sequence number) into respective transmission data packets, and transmits the transmission data packets to a receiver RLC. The receiver RLC then checks the SNs in the received data packets. If the SNs are not in sequence, or if there is a non-received data packet, the receiver RLC discards the received data packets though they are correctly received.
Second, a description will be made of an operation of the RLC in the AM. The sender RLC transmits data packets, into each of which a header with a SN is inserted, to the receiver RLC. The receiver RLC then checks the SNs in the received data packets. If the SNs are not sequential, or if there is a non-received data packet, the receiver RLC sends a retransmission request for the non-received data packet to the sender RLC. Upon receiving the retransmission request from the receiver RLC, the sender RLC retransmits a data packet corresponding to the non-received data packet to the receiver RLC.
FIG. 1 illustrates a process of resetting the RLC operating in the AM. To reset the RLC operating in the AM, HFNIs (Hyper Frame Number Indicators) of peer-to-peer RLCs, i.e., RLC of a UTRAN (UMTS Terrestrial Radio Access Network) and RLC of an UE are synchronized with each other, and after the synchronization, all the data blocks are discarded. The RLC reset procedure is started in a protocol error state, and after determining the RLC reset, a sender RLC 100 transmits a Reset PDU (Protocol Data Unit) to a peer-to-peer RLC, i.e., a receiver RLC 150 (Step 111). Upon receiving the Reset PDU, the receiver RLC 150 resets an RLC variable to an initial value and discards all the received PDUs. After performing the RLC reset process, the receiver RLC 150 transmits a Reset ACK PDU to the sender RLC 100 and ends the RLC reset process (Step 113).
Next, the RLC reset process by the sender RLC 100 and the receiver RLC 150 will be described with reference to a state transition diagram of FIG. 2.
FIG. 2 illustrates an RLC state transition diagram for the RLC reset process of FIG. 1. As illustrated in FIG. 2, in a Null state 200, the RLC can transmit no data. For data transmission, the RLC transits to an ACK Data Transfer Ready state 250 by reconfiguring an RLC in response to a control command from an RRC (Radio Resource Control) layer entity. In the following description, the RRC layer entity will be referred to as “RRC” for short. In the ACK Data Transfer Ready state 250, the RLC can exchange data blocks, and make transition to the Null state 200 in response to a control command from the RRC. When a protocol error occurs in the Null state 200, the RLC transmits an RLC Reset PDU to the counterpart RLC, and then makes transition to a Reset Pending state 270. In the Reset Pending state 270 also, the RLC cannot exchange data, and should receive an RLC Reset ACK PDU from the counterpart RLC in order to reset the state of the RLC and get out of the Reset Pending state 270.
This RLC reset process has been defined to cope with a protocol error in a common W-CDMA communication system not using the HSDPA. However, the use of the HSDPA causes unnecessary data transmission by the MAC. This is because when the HSDPA is used, a new MAC for supporting the HSDPA, i.e., MAC-hs is realized and the MAC-hs performs the HARQ function. That is, for transmission and retransmission of data blocks, the Node B performs a buffering function. Therefore, the data blocks transmitted by the RLC are buffered in the MAC-hs before being transmitted over the radio channel. At this point, if the RLC reset procedure is performed due to a protocol error occurring on the RLC, the data blocks buffered in the MAC-hs before the RLC reset are transmitted to the counterpart MAC-hs through the physical layer. However, when the counterpart MAC-hs, i.e., the receiver MAC-hs, receives the data blocks, the received data blocks are discarded in the receiver RLC by the RLC reset process. Therefore, while the RLC reset process is performed, the data block transmission by the MAC-hs is unnecessary. Further, until the RLC reset process is ended, the data block buffering causes unnecessary use of the memory. In addition, the receiver MAC-hs should also reset retransmission information, for normal operation. This is because when there exists a data block error-detected by the MAC-hs among the data blocks, or PDUs, received from the UTRAN, the MAC-hs should temporarily perform buffering in order to retransmit the error-detected data block. Therefore, the receiver MAC-hs unnecessarily uses the memory and this data block is also unnecessarily transmitted to the receiver RLC, an upper layer.